Control strategy in the vibrtion isolation system for KAGRA Ryutaro Takahashi (Institute for Cosmic Ray Research, Univ. of Tokyo / National Astronomical Observatory of Japan) The 3rd Korea-Japan workshop on KAGRA Sogang University, Seoul December, 2012

Contents 1. Vibration isolation system for KAGRA 2. Control system 3. Low frequency disturbance 1. Seismic noise 2. Ground strain 3. Thermal drift 4. Newtonian noise 4. Control strategy 1. Hierarchical contol 2. Common mode rejection 3. Feed-forward control 5. Consideration 6. Summary

1. Vibration isolation system for KAGRA

Top filter [Filter0] Inverted Pendulum (IP) Bottom Filter (BF) Test Mass (TM) Recoil Mass (RM) Pre-isolator Payload Filter chain Schematic view of Seismic attenuation system (SAS)Type-A/B Intermediate Mass (IM) Intermediate Recoil Mass (IRM) Geometric Anti-Spring (GAS) Filter1 (Filter1~3 in Type-A)

Type-A IP + GAS Filters (5 stages) + Payload (23kg, cryogenic) Type-B IP + GAS Filters (3 stages) + Payload (10kg/20kg) Type-C Stack + Single/Double-pendulum (~1kg) Disposition of vibration isolation system

Type-A (2-layer structure) Upper tunnel containing pre-isolator (short IP and top filter) Upper tunnel containing pre-isolator (short IP and top filter) 1.2m diameter 5m tall borehole containing standard filter chain 1.2m diameter 5m tall borehole containing standard filter chain Lower tunnel containing cryostat and payload Lower tunnel containing cryostat and payload 8m 5m 7m 1F 2F

Type-B IP base is supported by the outer frame. IP base is supported by the outer frame. Pre-isolator is the same as Type-A’s. Pre-isolator is the same as Type-A’s.

Type-B Payload Rigid table Type-C Payload Stack Type-C Type-B payload on rigid table

2. Contol system

Pre-isolator

Type-B Payload

Linear variable differencial transformer (LVDT) and Voice coil actuator Embeded LVDT- actuator unit on the inverted pendlum. Evaluation of LVDT. The noise level was less then 0.1  m at Hz. The noise level is proportional to DC offsets. Primaly coil Secondaly coil Yoke Coil Magnet VC LVDT

Evaluation of L4-C geophones in Kamioka. The noise level was 5x m Hz 1/2 at 1Hz. Test of geophones and preamplifirs in Kashiwa. Vertical Velocity responce of L4-C geophone. Ground 1 Ground 2 Differential Inertial sensor (Geophone)

Motor slider and Hydraulic leveler Level of the IP base is tuned by the tripodal hydraulic piston. Position of the IP is tuned by the motor sliders to compensate the DC component of the feedback signal to the actuator.

The sensitivity is ~ 2.5x m/Hz 1/2 at 1 Hz, and ~7x m/Hz 1/2 at 0.1 Hz. The linear range is ~ 1mm. Optical sensor and electro-magnetic actuator (OSEM)

Optical lever by K. Agatsuma Type-B

Control of IP (example of TAMA) PS Length ACC, LVDT  X ACC LVDT Actuator X Y  Global control of cavity Length after cavity lock Damping of excited torsion mode using Position Sensor

3. Low frequency disturbance

Peterson noise model Acceleration spectrum Dashed lines: Peterson high and low noise models Solid lines: noise spectral level for IRIS station (3 components) Global high (NHNM) and low (NLNM) noise models represent upper and lower bounds of a cumulative compilation of representative ground acceleration power spectral densities Seismic noise J. Havskov and G. Alguacil, “Instrumentation in Earthquake Seismology”, Springer, 2009

Origin of seismic noise Man made noise (Cultural noise) Originates from traffic and machinery with high frequencies (>2-4Hz). Propagates mainly as high-frequency surface waves which attenuate fast with distance and decrease strongly in amplitude with depth. Has a large difference between day and night. Wind noise Makes any object move. Usually high frequency, however large swinging objects can generate lower frequency signals. Horizontal noise attenuation (dB, spectral acceleration density) as a function of depth and period Ocean generated noise (microseisms, microseismic noise) Seen globally. Long period (10~16s): generated only in shallow waters in coastal region. Shorter period (peak~5s): generated by the superposition of ocean waves of equal period traveling in opposite directions Seismic noise

3-2. Ground strain JGW-G by A. Araya Tidal variation

3-3. Thermal drift R. Takahashi, et al : Rev. Sci. Instrum. 73 (2002) 2428 Stack isolation in Type-C system

Naive estimation Naive estimation Generally this noise is smaller in underground. Generally this noise is smaller in underground. Large amount of moving water due to melted snow may make effective noise in spring around Kamioka. Large amount of moving water due to melted snow may make effective noise in spring around Kamioka Newtonian noise Ambient seismic waves induce density perturbations, which produce fluctuating gravitational forces. Estimated Newtonian noise in LIGO (Hughes and Thorne, PRD )

4. Control strategy

4-1. Hierarchical control ActuationSensingControl Band Moter Slider on IP Offset of VC 1/day Voice Coil on IP LVDT<0.1Hz Geophone0.1-1Hz Global<0.1Hz Intermediate Mass OSEM<1Hz Global0.1-1Hz Test MassGlobal1-1kHz ActuationSensingControl Band Moter Slider on Filter0 Offset of VC 1/day Voice Coil on Filter0 LVDT<1Hz Voice Coil on Filter1-3 LVDT0.1-1Hz Intermediate Mass OSEM<1Hz Displacement horizontalvertical

4-1. Hierarchical control ActuationSensingControl Band Hydraulic leveler on IP Offset of VC 1/day Moter Slider on IM Offser of TM 1/day Intermediate Mass OSEM<1Hz Test MassOptical Lever <1Hz Global<0.1Hz ActuationSensingControl Band Moter Slider on Filter0 Offset of TM 1/day Voice Coil on IP LVDT<0.1Hz Geophone0.1-1Hz Intermediate Mass OSEM<1Hz Test MassOptical Lever <1Hz Global<0.1Hz Angle pitchyaw

4-2. Common mode rejection (CMR) The interferometer senses not local displacement but length between mirrors. The interferometer senses not local displacement but length between mirrors. The mirrors move in common mode at low frequencies. The mirrors move in common mode at low frequencies. Microseismic noise was reduced by the common mode rejection in TAMA or CLIO. Microseismic noise was reduced by the common mode rejection in TAMA or CLIO. Such a reduction is not expected in the 3-km cavity of KAGRA. Such a reduction is not expected in the 3-km cavity of KAGRA.

Witness sensor is effected by the feed-back control. Witness sensor is effected by the feed-back control. Witness sensor is NOT effected by the feed-forward control. Witness sensor is NOT effected by the feed-forward control Feed-forward control Witness Sensor Filter Witness Sensor Filter Target signal Target signal Witness Sensor Filter Target signal feed-back feed-forward feed-forward (offline) Comparision with feed-back control

5. Consideration There are many kinds of servo loops. Control bands are limited by the sensing noises. There are many kinds of servo loops. Control bands are limited by the sensing noises. A large disturbance at low frequencies should be fedback to the upper reaches considering phase delay. A large disturbance at low frequencies should be fedback to the upper reaches considering phase delay. Sensing signals must be diagonalized well for independent controls. Sensing signals must be diagonalized well for independent controls. CMR at the low frequencies is not effective in the length control of the 3-km cavity, it is expected only in the center area. CMR at the low frequencies is not effective in the length control of the 3-km cavity, it is expected only in the center area. Feed-forward control is useful in the cace of having independent monitor like seismometer (seismic noise), strain meter (tidal variation), or gravity gradiometer (Newtonian noise). Feed-forward control is useful in the cace of having independent monitor like seismometer (seismic noise), strain meter (tidal variation), or gravity gradiometer (Newtonian noise).

6. Summary KAGRA employed the SAS which consists of an inverted pendulum and geometric anti-spring filters. KAGRA employed the SAS which consists of an inverted pendulum and geometric anti-spring filters. There are some kinds of displacement noise sources at frequencies lower than 1Hz as well as observation band. There are some kinds of displacement noise sources at frequencies lower than 1Hz as well as observation band. The vibration isolation system is controled by multiple servo loops using many kinds of sensors and actuators. The vibration isolation system is controled by multiple servo loops using many kinds of sensors and actuators. Hierarchical control is required considering sensing noises, CMR and feed-forward loops. Hierarchical control is required considering sensing noises, CMR and feed-forward loops.